An anticipated development of broadband access networks is for Access Nodes (AN's), providing high speed broadband connections to end users via short twisted copper pair telephone lines, to be located at points (e.g. drop points) having no easy access to a mains power supply, and being connected to an Exchange building via an optical connection. In such a deployment, one possible known way of providing electrical power to the AN's is for Customer Premises Equipment (CPE) devices to include a power supply unit known as a reverse power supply unit which is designed to provide power to the AN over the same twisted copper pair used to transmit data to and from the CPE device and the AN. The International Telecommunication Union (ITU) is developing a standard for such an approach under the project name G.Fast.
In such arrangements, the reverse power supply unit often supplies the power by means of a direct current (dc) constant voltage supply from which the AN can draw up to a predetermined amount of electricity for its power needs. Such systems also typically include an Analog Telephone Adapter (ATA) unit which enables conventional telephone devices which expect a standard “analog” interface to the telephone network (e.g. to the Public Switched Telephone Network (PSTN) in the UK via a conventional PSTN “British Telecom socket” adapted to receive a conventional BS 6312 jack also known as a “British Telecom plug” or via an RJ11 type plug, etc.) to operate in such a system. The ATA unit typically provides a socket into which the conventional telephone device can connect and the ATA simulates the behavior of a conventional analog interface to a telephone network such as the British PSTN.
Prior patent applications of the present applicant (see copending published PCT applications WO2012069788 and WO2012032309) describe systems including special “dongle” devices which can operate in such a system to enable conventional devices to be connected to a user's existing extension sockets (via suitable dongles). In such an environment, the user's extension sockets and master socket (customer/user premises side) and the wiring between these sockets form the user's internal “home wiring”. The dongles operate (together with a cooperating specially adapted ATA unit) to convert any signals or other behavior (e.g. a telephone device presenting a low dc resistance to the socket when transitioning to an “off-hook” state) which are inappropriate for transmission over wiring which is carrying a reverse power supply, into or from alternative signals which are appropriate for carrying over such wiring (or they may act to switch such signals onto or from alternative physical channels—for example spare additional wires within the user's home wiring). In this way, the prior inventions permit the existing home wiring to be re-used and all that is required is that the ATA device and the conventional telephones (these latter via the special dongles) are connected to the user's existing home wiring via the sockets provided in the user's existing home wiring.
Although most conventional telephone devices in use today have their own power supply, the conventional PSTN interface is still able to supply a modest amount of power as a legacy from earlier telephone devices which did not have their own power supply and had to draw all of their power needs from the public telephone network, the largest power requirement being that needed to generate a ring (originally by means of an electro-mechanical bell). Conventional telephone devices still expect therefore to be able to draw a modest amount of power from the interface to which they expect to connect via the telephone lead, and expect that if they present a low dc interface (the typical behavior when the device transitions to an off-hook state) they will receive a small current (sufficient to operate a conventional unpowered telephone device). Any dongles (or other arrangement) therefore which permit a user to connect a conventional telephone device to the user's home wiring in a case where a reverse power system is in place between a user premises and an AN that requires reverse powering should be designed to permit such a small amount of power to be drawn, but no more, even when the device presents a low dc resistance.
A potential problem with such an arrangement, however, is that a user may incorrectly connect a conventional telephone device to the user premises home wiring not via a suitable dongle, or in some other uncontrolled way which means that the device is directly connected onto wiring which is providing a reverse power to an AN (which is also connected to that wiring but is remote from the reverse power supply unit). In such a case, the device could present a low dc resistance to the home wiring (and therefore the reverse power supply unit) with the result that the conventional device draws such a large current that the device is damaged, or, in a worst possible case, becomes sufficiently hot that it becomes a fire hazard, etc.
ETSI Contribution document ATTMTM6(13)000089 “Reverse Power Feed start-up protocol” by Christophe Gendarme and Francois Fredricx both of Alcatel-Lucent, dated 23 Aug. 2013, describes a proposed procedure for guaranteeing proper interaction between the key elements of a Reverse Power Feed system. The document specifies that the Power Source Equipment (PSE) should “continuously check for directly connected (without adaptor) off-hook phones on the home network and back off in case of detection.” It also specifies that during start up of reverse powering, “The PSE first checks that there is no off-hook phone, signals that it requests to apply RPF, and waits for the disappearing of any voltage on the line (covering the POTS case) and then the presence of a ready signaling from the DPU (covering the case without POTS) before applying RPF.” It then goes on to further specify that “The low power mode prior to the full RPF applies a voltage and current that is safe for directly connected off-hook phones and serves two goals; detection of off-hook phones (new line) provide some bootstrap power to the DPU . . . . Checking for presence of off-hook phones must be continued during the normal RPF operation.” From the above comments, it is clear that this document addresses the same issue as addressed by the present disclosure but it does not specify how to determine whether a directly (i.e. not via a dongle) connected telephone device is connected to the home wiring set over which a reverse power feed is being supplied or not.